How many letters does the top answer for “use of physical or mental energy; hard work” have?

The most common answer is exertion (8 letters).

Crossword clue: use of physical or mental energy; hard work

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

exertion 8 letters wn-def

use of physical or mental energy; hard work

travail 7 letters wn-def

use of physical or mental energy; hard work